Observation of Charge Density Wave in Layered Hexagonal Cu1.89Te Single Crystal

We report comprehensive transport, electron microscopy and Raman spectroscopy studies on transition-metal chalcogenides Cu1.89Te single crystals. The metallic Cu1.89Te displays successive metal-semiconductor transitions at low temperatures and almost ideal linear MR when magnetic field up to 33 T. Through the electron diffraction patterns, the stable room-temperature phase is identified as a 3 × 3 × 2 modulated superstructure based on the Nowotny hexagonal structure. The superlattice spots of transmission electron microscopy and scanning tunneling microscopy clearly show the structural transitions from the room-temperature commensurate Ⅰ phase, named as C-Ⅰ phase, to the low temperature commensurate Ⅱ(C-Ⅱ) phase. All the results can be understood in terms of charge density wave(CDW) instability, yielding intuitive evidences for the CDW formations in Cu1.89Te. The additional Raman modes below room temperature further reveal that the zone-folded phonon modes may play an important role on the CDW transitions. Our research sheds light on the novel electron features of Cu1.89Te at low temperature, and may provide potential applications for future nano-devices.

巧妙的稀掺杂策略实现高性能GeTe热电材料

In thermoelectrics,doping is essential to augment the figure of merit.Traditional strategy,predomina ntly heavy doping,aims to optimize carrier concentration and restrain lattice thermal conductivity.However,this tactic can severely hamper carrier transport due to pronounced point defect scattering,particularly in materials with inherently low carrier mean-free-path.Conversely,dilute doping,although minimally affecting carrier mobility,frequently fails to optimize other vital thermoelectric parameters.Herein,we present a more nuanced dilute doping strategy in GeTe,leveraging the multifaceted roles of small-size metal atoms.A mere 4%CuPbSbTe_(3)introduction into GeTe swiftly suppresses rhombohedral distortion and optimizes carrier concentration through the aid of Cu interstitials.Additionally,the formation of multiscale microstructures,including zero-dimensional Cu interstitials,one-dimensional dislocations,two-dimensional planar defects,and three-dimensional nanoscale amorphous GeO_(2)and Cu_(2)GeTe_(3)precipitates,along with the ensuing lattice softening,contributes to an ultralow lattice thermal conductivity.Intriguingly,dilute CuPbSbTe_(3)doping incurs only a marginal decrease in carrier mobility.Subsequent trace Cd doping,employed to alleviate the bipolar effect and align the valence bands,yields an impressive figure-of-merit of 2.03 at 623 K in(Ge_(0.97)Cd_(0.03)Te)_(0.96)(CuPbSbTe_(3))_(0.04).This leads to a high energyconversion efficiency of 7.9%and a significant power density of 3.44 W cm^(-2)at a temperature difference of 500 K.These results underscore the invaluable insights gained into the constructive role of nuanced dilute doping in the concurrent tuning of carrier and phonon transport in GeTe and other thermoelectric materials.

Enhanced thermoelectric performance and mechanical strength in GeTe enable power generation and cooling

Finding a real thermoelectric(TE)material that excels in various aspects of TE performance,mechanical properties,TE power generation,and cooling is challenging for its commercialization.Herein,we report a novel multifunctional Ge0.78Cd0.06Pb0.1Sb0.06Te material with excellent TE performance and mechanical strength,which is utilized to construct candidate TE power generation and cooling devices near room temperature.Specifically,the effectiveness of band convergence,combined with optimized carrier concentration and electronic quality factor,distinctly boosts the Seebeck coefficient,thus greatly improving the power factor.Advanced electron microscopy observation indicates that complex multi-scale hierarchical structures and strain field distributions lead to ultra-low lattice thermal conductivity,and also effectively enhance mechanical properties.High ZT0.6 at 303 K,average ZTave1.18 from 303 to 553 K,and Vickers hardness of200 Hv in Ge0.78Cd0.06Pb0.1Sb0.06Te are obtained synchronously.Particularly,a 7-pair TE cooling device with a maximumΔT of45.9 K at Th=328 K,and a conversion efficiency of5.2%at Th=553 K is achieved in a single-leg device.The present findings demonstrate a unique approach to developing superior multifunctional GeTe-based alloys,opening up a promising avenue for commercial applications.

Manipulation of Band Degeneracy and Lattice Strain for Extraordinary PbTe Thermoelectrics

Maximizing band degeneracy and minimizing phonon relaxation time are proven to be successful for advancing thermoelectrics.Alloying with monotellurides has been known to be an effective approach for converging the valence bands of PbTe for electronic improvements,while the lattice thermal conductivity of the materials remains available room for being further reduced.It is recently revealed that the broadening of phonon dispersion measures the strength of phonon scattering,and lattice dislocations are particularly effective sources for such broadening through lattice strain fluctuations.In this work,a fine control of MnTe and EuTe alloying enables a significant increase in density of electron states near the valence band edge of PbTe due to involvement of multiple transporting bands,while the creation of dense in-grain dislocations leads to an effective broadening in phonon dispersion for reduced phonon lifetime due to the large strain fluctuations of dislocations as confirmed by synchrotron X-ray diffraction.The synergy of both electronic and thermal improvements successfully leads the average thermoelectric figure of merit to be higher than that ever reported for p-type PbTe at working temperatures.

In Situ Investigation of the Phase Transition at the Surface of Thermoelectric PbTe with van der Waals Control

The structure of thermoelectric materials largely determines the thermoelectric characteristics.Hence,a better understanding of the details of the structural transformation process/conditions can open doors for new applications.In this study,the structural transformation of PbTe(a typical thermoelectric material)is studied at the atomic scale,and both nucleation and growth are analyzed.We found that the phase transition mainly occurs at the surface of the material,and it is mainly determined by the surface energy and the degree of freedom the atoms have.After exposure to an electron beam and high temperature,highdensity crystal-nuclei appear on the surface,which continue to grow into large particles.The particle formation is consistent with the known oriented-attachment growth mode.In addition,the geometric structure changes during the transformation process.The growth of nanoparticles is largely determined by the van der Waals force,due to which adjacent particles gradually move closer.During this movement,as the relative position of the particles changes,the direction of the interaction force changes too,which causes the particles to rotate by a certain angle.